I am in the process of buying a used Merc. I was told it came with two keys. Later on I was told the second key is on it's way as it was still with the previous owner. Now I am told there is no spare key.
Paul - I had a very similar issue when I bought my one-previous-owner CLK two years ago - also in the UK.
In my case the existence of only one key was not revealed until I had handed over my banker's cheque, and I was being given all the paperwork, etc. The dealer's story was that they would chase the original owner for the 2nd key, and I trusted them on that, but had it written onto the purchase order.
Several months passed, and many phone calls were made, all to no avail as the first owner (diplomat) had now returned to his own country. Finally I bought a key myself, as I felt very exposed without a spare, and sent them an invoice for the cost. More wrangling, until finally I accepted a 50:50 deal. Otherwise I could see myself getting nothing.
One of my fears was that the PO might have given his key to a colleage, and my car could be traced and then stolen - it has a tracker installed.
However the MB dealership knew how to cancel the missing key from operation. You do know that they can only supply 6 keys before you have to have all your locks changed, don't you? That's if it is a recent model.